1. Field of the Invention
The present invention relates, in general, to a catalyst for the disproportionation/transalklylation of aromatic hydrocarbons and, more particularly, to a catalyst which is useful to prepare mixed xylenes from a mixture of benzene, toluene and C9 or higher aromatic compounds through disproportionation/transalklylation. Also, the present invention is concerned with a method for preparing such a catalyst.
2. Description of the Prior Art
Mixed xylenes, very important raw materials in the petrochemical industry, are composed of ethyl benzene and xylene isomers including meta-xylene, para-xylene and ortho-xylene. From mixed xylenes, thus, pure m-xylene, p-xylene and o-xylene, all important base fractions in the petrochemical industry, can be produced.
Owing to their similar boiling points, xylene isomers are very difficult to individually extract from the mixed xylenes by ordinary distillation processes. Usually, adsorptive separation, crystallization and/or isomerization are used to separate the individual isomers from the mixed xylenes.
To produce the mixed xylenes, the separation from the fractions fractions rich in the mixed xylenes and the synthesis by reaction are commercially utilized. The former includes the separation from the reformed oils produced during the reforming of naphtha and the separation from the thermally cracked oils which are produced as by-products during thermal cracking. As for the latter, it can be exemplified by the disproportionation of toluene, the transalkylation of toluene/C9 aromatic hydrocarbons, and the alkylation of toluene with methanol.
The disproportionation/transalkylation catalysts which are used for commercial purposes are usually based on zeolites, such as mordenite and ZSM-5. For use, these zeolites are either molded or impregnated with catalytic metal components.
In many prior patents are found zeolite-based disproportionation/transalkylation catalysts.
U.S. Pat. No. 4,083,886 discloses a catalyst for transalkylating alkyl aromatic compounds such as toluene, which is prepared by molding mordenite with an inorganic oxide binder after being treated with aqueous ammonia. In contrast with the present invention, this catalyst does not employ a metal capable of hydrogenation, but performs transalkylation by zeolite itself. Toluene alone can be smoothly transalkylated by this catalyst. From a reaction containing C9 or higher aromatic hydrocarbons, however, a high yield of the mixed xylenes cannot be expected because the reaction is hard to dealkylate with the catalyst. In addition, the catalyst is deactivated faster as the proportion of C9 or higher aromatic hydrocarbons becomes larger.
U.S. Pat. No. 5,030,787 discloses a transalkylating catalyst which is based on beta-zeolite whose acidity is weakened by a steam treatment. With the intent to reduce side products and to retard the deactivation of catalyst, the weakening of acidity is conducted. However, the weakening of the acidity without the introduction of a hydrogenating metal results in deteriorating the catalytic activity of the catalyst and thus, reducing the yield of the mixed xylenes.
U.S. Pat. No. 5,475,180 pertains to the disproportionation/transalkylation of toluene and high molecular weight aromatic hydrocarbons by use of a catalyst comprising nickel supported on mordenite. As in the present invention, the nickel functions to effectively dealkylate the aromatic hydrocarbons of large molecular weights and to suppress the deactivation of the catalyst. However, it is expected that this catalyst becomes deactivated faster during the catalytic reaction than does the catalyst employing activity-controlled platinum because the hydrogenation activity of nickel is far poorer than that of platinum. In addition, when sulfur compounds are introduced into the reaction, the hydrogenation activity of the nickel is greatly deteriorated owing to its strong association with the sulfur compounds. The feedstocks, which are fed into the commercial disproportionation/transalkylation process, usually undergo desulfurization in advance, but there is not completely excluded the possibility that sulfur compounds might flow into the disproportionation/transalkylation process owing to process accidents or operational errors. Upon the flowing in of sulfur compounds, the platinum catalyst has its platinum ingredient adsorbed with the sulfur compounds and thus, becomes deactivated temporarily. When the influx of sulfur compounds are stopped, however, the catalyst restores its catalytic activity because the sulfur compounds are desorbed from the platinum. In the case of nickel, the sulfur compounds, if adsorbed once, are very difficult to remove during reaction.
Another catalyst for the disproportionation/transalkylation of toluene and C9 aromatic hydrocarbons is found in U.S. Pat. No. 3,671,602 which discloses an alkali metal-deficient mordenite on which aluminum fluoride and a metal selected from the group consisting of Cu, Ag and Au or from the group consisting of W, Mo, Cr and As are supported, affirming that aluminum fluoride plays a role in restraining the production of coke so as to inhibit the deactivation of the catalyst. In this case the anti-deactivation effect cannot be efficiently conducted when the reaction contains C10 aromatic hydrocarbons or a high proportion of C9 aromatic hydrocarbons.
Also, the disproportionation/transalkylation of toluene and alkyl aromatic hydrocarbons is described in U.S. Pat. No. 4,723,048 which discloses a catalyst comprising mordenite on which a metal of Group VIII, such as nickel or palladium, a metal of Group IB, such as Ag, and a metal of Group IVA, such as Sn, Pb or Ge, are supported. In this patent, the metal of Group VIII, such as nickel or palladium, serves as a hydrogenating metal whose activity is controlled by the metal of Group IVA, thereby improving the performance of the catalyst. The metals such as nickel and palladium are significantly poor in hydrogenation activity as compared with platinum, used in the present invention. When the reaction gets a high content of C9 or higher aromatic hydrocarbons, the catalyst is difficult to protect from deactivation. In addition, the presence of sulfur compounds in the reaction may make the hydrogenating activity of the catalyst drop to an unrecoverable state.
U.S. Pat. No. 5,475,179 discloses a catalyst for the disproportionation of toluene, which is based on Si-treated ZSM-5 type zeolite. It is also described that the treatment of ZSM-5 type zeolite with silicon makes the shape selectivity of the ZSM-5 type zeolite increase, so that the selectivity for the p-xylene of the mixed xylenes produced upon the disproportionation of toluene comes to reach about 90 wt % which is far higher than the thermodynamic equilibrium, 24 wt %. The use of ZSM-5 type zeolite alone, however, can be applied for toluene only, but cannot be applied for the disproportionation or transalkylation of C9 or higher aromatic hydrocarbons because of its structural limit.
In addition, there are many examples of the disproportionation/transalkylation catalysts using mordenite, or beta type or ZSM-5 type zeolite as a base material, but nowhere is found a catalyst which uses platinum as a hydrogenating function and tin or lead as an activity controller so as to bring about a great improvement in the yield of mixed xylenes and in the deactivation of catalyst, as in the present invention.